Sensor chip with additional heating element, method for preventing a sensor chip from being soiled, and use of an additional heating element on a sensor chip

ABSTRACT

In the case of a sensor chip according to the related art, impurities in a flowing medium may be deposited in the area of a sensor area and may permanently soil it, resulting in a negative effect on the measurement response of the membrane.  
     In the case of the novel sensor chip ( 1 ), an additional heater ( 39 ) is situated upstream and at a definite distance from the sensor area, resulting in the impurities in the flowing medium being deposited in the area of the additional heater ( 39 ), i.e., in their being unable to reach the sensor area ( 17 ).

BACKGROUND INFORMATION

[0001] The present invention is directed to a sensor chip according tothe preamble of claim 1 and use of an additional heater on a sensor chipaccording to the preamble of claim 2 and a method of preventingcontamination on a sensor chip according to the preamble of claim 3.

[0002] German Patent Application 196 01 791 A1 describes a sensor chiphaving a sensor area composed of a frame element, a recess, and amembrane, for example. An unwanted influence on the measuring signal ofthe sensor chip in the sensor area may occur repeatedly due tocontamination, e.g., oil, to which the sensor chip is exposed.Contamination with oil in the sensor area or in the immediate areaaround the sensor area alters the thermal conductivity at the surface ofthe sensor chip and thus affects the measuring signal. In addition, oildeposited on the sensor chip forms an adhesive for particles containedin a flowing medium. These trapped particles further increase theunfavorable effect.

[0003] U.S. Pat. No. 5,705,745 describes a sensor chip having a membraneon which are arranged temperature resistors and heating resistors, themembrane being surrounded by a thermally conductive element, which mayalso be U-shaped. The thermally conductive element is not heated. Thethermally conductive element is also situated at least partially in thearea of the membrane.

[0004] U.S. Pat. No. 4,888,988 describes a sensor chip having amembrane, a metallic conductor being situated around the membrane butnot in the area of the membrane. This conductor is the common groundedneutral conductor of the measuring arrangement on the sensor chip. Thecross section of this grounded neutral conductor has even been increasedselectively to prevent an increase in temperature. An elevatedtemperature of the grounded neutral conductor would also have anextremely deleterious effect on the measurement according to thismethod.

[0005] German Patent Application 198 01 484 A1 describes a sensor chiphaving a membrane, electric conductors being situated around themembrane with an electric current flowing through them. These conductorsare temperature sensors which are used for the measurement method and/orthe measurement procedure.

[0006] German Patent Application 2 900 210 A1 and U.S. Pat. No.4,294,114 describe a sensor chip having a temperature-dependent resistoron a carrier, another resistor directly adjacent to thetemperature-dependent resistor also being applied to the carrier.

[0007] German Patent Application 4 219 454 and U.S. Pat. No. 5,404,753describe a sensor chip having a reference temperature sensor at adistance from a sensor area.

[0008] German Patent Application 3 135 793 A1 and U.S. Pat. No.4,468,963 describe a sensor chip having another resistor upstream and/ordownstream from the sensor resistor, but the additional resistorinfluences the measuring signal.

ADVANTAGES OF THE INVENTION

[0009] The sensor chip according to the present invention and the use ofan additional heater on a sensor chip according to the present inventionand the method according to the present invention for preventingcontamination of a sensor chip having the characterizing features ofclaims 1, 2, and 3, respectively, has the advantage over the related artthat contamination of the sensor area of the sensor chip is reduced orprevented in a simple way.

[0010] Advantageous refinements of and improvements on the sensor chipand the aforementioned use and the aforementioned method ascharacterized in claims 1, 2, and 3, respectively, are possible throughthe measures characterized in the dependent claims.

[0011] An additional heater situated at a distance of up to 1 mm fromthe membrane is advantageous, so that the precipitates which areintentionally formed there are far enough away from the sensor area andare unable to influence the measurement response of the sensor area.

[0012] The additional heater advantageously is in a U shape enclosingthe sensor area in an advantageous manner.

DRAWING

[0013] Exemplary embodiments of the present invention are shown insimplified form in the drawing and are explained in greater detail inthe following description.

[0014]FIG. 1 shows a sensor chip according to the related art;

[0015]FIG. 2a shows a first exemplary embodiment of a sensor chipaccording to the present invention; FIG. 2b shows a second embodiment,and FIG. 2c shows a third embodiment;

[0016]FIGS. 3a and 3 b show a sensor chip designed according to thepresent invention and a control circuit; and

[0017]FIG. 4 shows a temperature profile of an additional heater and asensor area of a sensor chip designed according to the presentinvention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0018]FIG. 1 shows a sensor chip according to the related art which isimproved according to the present invention according to thedescriptions of FIGS. 2a through 2 c. The production method and the useof such a sensor chip are described in detail in German PatentApplication 196 01 791 A1, which is herewith explicitly to be includedas part of the present disclosure.

[0019] The sensor chip has a frame element 3 made of silicon forexample. Frame element 3 has a recess 5. A dielectric layer 21, e.g.,made of SiO₂, for example, is applied to the frame element. Layer 21 mayextend over entire frame element 3 or only over an area of recess 5.This area forms a membrane area 7 which partially or entirely delimitsrecess 5 on one side.

[0020] At least one, e.g., three metal strips 19 are applied on the sideof membrane area 7 facing away from recess 5. Metal strips 19 formelectric heaters and/or measuring shunts, for example, forming a sensorarea 17 together with membrane area 7. Sensor area 17 is preferablycovered with a protective layer 23. Protective layer 23 may also extendonly over metal strips 19.

[0021] Membrane area 7 is then formed in part by dielectric layer 21producing a measuring signal, a membrane 33 and in part by protectivelayer 21. The sensor chip has a surface 27 which is in direct contactwith a flowing medium.

[0022]FIG. 2a shows a top view of a first exemplary embodiment of asensor chip 1 designed according to the present invention. Sensor chip 1has a sensor area 17 having a length 1 across a main direction of flow42. For example, metal strips 19 which form at least one electricheating resistor 35 and at least one temperature sensor 37, for example,are situated in sensor area 17. Temperature sensor 37 is also anelectric resistor, for example. In this case there will be one heatingresistor 35 and two temperature sensors 37. Metal strips 19 are mostlysituated in sensor area 17 and are the prerequisite for a measurementmethod for determining at least one parameter, e.g., the temperature andflow rate of the flowing medium. Sensor area 17 is therefore connectedto a control and regulating circuit. Sensor area 17 may be formed bymembrane 33 described above, for example. Sensor chip 1 is situated in aflowing medium for determination of at least one parameter, the flowingmedium flowing in the main direction of flow 42, on or over sensor chip1, i.e., surface 27. The flowing medium may contain impurities which mayresult in contamination of sensor chip 1. These include, for example,salts dissolved in water or oil. To prevent these impurities from beingdeposited in the area of sensor area 17, at least partially anadditional heater 39 is situated upstream from sensor area 17, forexample, is connected to a current source (not shown), and is heated byits ohmic resistance. Additional heater 39 is situated at a defineddistance, e.g., up to 1 mm away from sensor area 17.

[0023] No control circuit is necessary to regulate the temperature ofadditional heater 39. An amperage determined by the design, i.e., by thecross section is sufficient. Additional heater 39 is not used for ameasurement method for determining a parameter of the flowing medium,i.e., it is not a component of this measurement zone.

[0024] Additional heater 39 here is in the form of a straight line, forexample, which extends across, e.g., perpendicular to, the maindirection of flow 42, e.g., extending beyond a length 1 of sensor area17. Additional heater 39 may also have a spiral shape. Due to additionalheater 39, contamination of sensor chip 1 occurs in the area ofadditional heater 39, but at a definite distance away from sensor area17 so that the measurement response of sensor area 17 is not affected.This contamination is thus displaced from sensor area 17 into the areaaround additional heater 39.

[0025] The temperature of additional heater 39 is set so that there is asharp temperature transition in the area of additional heater 39, sothat thermal gradient eddies are produced, more or less filtering theliquid or the oil out of the flowing medium, i.e., the heaviercomponents of the flowing medium are deposited on surface 27 in the areaof additional heater 39 but not in sensor area 17.

[0026]FIG. 2b shows a top view of another exemplary embodiment of sensorchip 1 according to the present invention. In contrast with FIG. 2a,additional heater 39 is U shaped. The U shape of additional heater 39 isin turn situated on sensor chip 1 at a definite distance away upstreamfrom sensor area 17, the two legs of the U shape running across maindirection of flow 42.

[0027]FIG. 2c shows a top view of another exemplary embodiment of asensor chip 1 according to the present invention. Additional heater 39again has a U shape which at least partially encloses sensor area 17.Additional heater 39 runs on downstream and upstream sides, for example,definitely at a distance from sensor area 17 and on an end face ofmembrane 33.

[0028] Additional heater 39 is designed, for example, so that it has alength greater than that of sensor area 17, for example, at leastupstream or downstream from sensor area 17. Therefore, sensor area 17 isprotected from contamination over its entire length 1.

[0029] Resistors 35, 37 and/or additional heater 39 are preferablydesigned as printed conductors.

[0030] Sensor chip 1 is designed in the form of a chip, for example andhas surface 27 past which the flowing medium flows. Sensor area 17 andadditional heater 39 are situated together on surface 27.

[0031]FIG. 3a shows a sensor chip 1 designed according to the presentinvention, having a sensor area 17 and a first control circuit 54 whichis electrically connected to sensor area 17 by electric conductors 51,e.g., bond wires. First control circuit 54 has a first power source 45,e.g., a current or voltage source, or it is connected electrically tosuch a source by which at least one heating resistor 35 or at least onetemperature sensor 37 is heated electrically in sensor area 17.

[0032] Additional heater 39 is connected to a separate second powersource 48, for example, via electric conductors 51. There is no electricconnection between first control circuit 54 and second power source 48.First control circuit 54 thus supplies a measuring signal, e.g., for anengine controller which is independent of operation of additional heater39, i.e., the operation of additional heater 39 has no effect on themeasuring signal. First power source 45 may also heat additional heater39, e.g., via a voltage splitter, but the control signal of first powersource 45 to additional heater 39 is still independent of themeasurement method or signals to sensor area 17.

[0033] Sensor chip 1 supplies a measuring signal, e.g., for regulatingan internal combustion engine. Additional heater 39 for example isheated only when the engine is not in operation, because only aftershutdown of the engine does the most frequent contamination of sensorchip 1 occur due to backflow, e.g., from crankcase venting, containingcontaminants such as oil. First control circuit 54 may deliver thesignal for the heating operation of additional heater 39, for example,by closing a switch 60, for example, so that second power source 48heats additional heater 39.

[0034] This control signal for heating additional heater 39 when theengine is not in operation may also be supplied by a second controlcircuit 57. Second control circuit 57 is the engine regulating unit, forexample (FIG. 3b).

[0035]FIG. 4 shows a temperature profile of additional heater 39 andsensor area 17. FIG. 4 shows an X/Y diagram, a length in main directionof flow 42 being plotted on the X axis and a temperature on the surfaceof sensor chip 1 being plotted on the Y axis.

[0036] Additional heater 39 is located, for example, upstream fromsensor area 17. Between additional heater 39 and sensor area 17 there isa distance which is different from zero. For example, the resistors insensor area 17 generate a trapezoidal temperature curve having a maximumtemperature T_(M).

[0037] Additional heater 39 has a maximum temperature T_(Z) which variesaccording to a parabolic curve, for example, which is equal to orgreater than temperature T_(M).

[0038] Arrows 62 show the flow pattern of the medium near surface 27.Additional heater 39 creates a more or less abrupt increase intemperature at surface 27, i.e., a thermal gradient which is large anddiffers from zero. Oncoming particles near surface 27 are more or lesssucked by a partial vacuum to surface 27 upstream from or at the initialarea of additional heater 39, and then rise upward in the area of theadditional heater, i.e., removing themselves from surface 27. Due tothis flow pattern, thermal gradient eddies 65 are created in the area ofadditional heater 39. Particles of dirt or oil therefore adhere tosurface 27 of sensor chip 1 in the area of additional heater 39, so thatthe flowing medium is cleaned in the area near the surface, and sensorarea 17 has little or no contamination.

What is claimed is:
 1. A sensor chip for measuring at least oneparameter of a flowing medium, having a sensor area for at least onemeasurement method, wherein at least one additional heater (39) issituated on the sensor chip (1) at a distance from the sensor area (17),and the sensor area (17) is operated independently of the additionalheater (39).
 2. A use of at least one additional heater (39) for formingthermal gradient eddies (65) in a flowing medium in the area of theadditional heater (39), the additional heater (39) for determining atleast one parameter of a flowing medium being situated on a sensor chip(1) and at a distance from a sensor area (17) of the sensor chip (1). 3.A method of preventing contamination on a sensor chip (1) which has asensor area (17) and is situated in a flowing medium, wherein at leastone additional heater (39) is heated electrically due to its ohmicresistance so that thermal gradient eddies are formed in the area of theadditional heater (39), resulting in precipitation of the contaminantsin the flowing medium in the area of the additional heater (39), awayfrom the area of the sensor area (17).
 4. The sensor chip as recited inclaim 1, wherein the sensor area (17) is electrically connected to afirst control circuit (54) which generates a measuring signal which isindependent of the operation of the additional heater (39).
 5. Thesensor chip as recited in claim 4, wherein the first control circuit(54) controls a first energy source (45), and the additional heater (39)is electrically connected to a separate second energy source (48). 6.The sensor chip as recited in claim 1, 4 or 5, wherein the additionalheater (39) is at least partially at a distance of up to 1 mm away fromthe sensor area (17).
 7. The sensor chip as recited in one or more ofclaims 1 or 4 through 6, wherein the additional heater (39) is U shaped.8. The sensor chip as recited in claim 7, wherein the U shape at leastpartially encloses the sensor area (17).
 9. The sensor chip as recitedin claim 1, wherein the flowing medium has a main direction of flow (42)and the additional heater (39) is situated at least partially in themain direction of flow (42), upstream from the sensor area (17).
 10. Thesensor chip as recited in claim 1, wherein the sensor area (17) has amembrane (33).
 11. The sensor chip as recited in claim 1, wherein atleast one heating resistor (35) and at least one temperature sensor (37)are situated in the sensor area (17) which (35, 37) are situated mostlyin the sensor area (17).
 12. The sensor chip as recited in claim 1,wherein the flowing medium has a main direction of flow (42); and theadditional heater (39) is situated at least partially in the maindirection of flow (42) downstream from the sensor area (17).
 13. Thesensor chip as recited in claim 1, wherein the flowing medium has a maindirection of flow (42); and the sensor area (17) has a length (1) acrossthe main direction of flow (42); and the additional heater (39) issituated across the main direction of flow (42) and is longer than thelength (1).
 14. The sensor chip as recited in claim 11, wherein theresistors (35) or the temperature sensor (37) are designed as printedconductors.
 15. The sensor chip as recited in one or more of claims 1, 6or 7, wherein the additional heater (39) is designed as a printedconductor.
 16. The sensor chip as recited in claim 1, wherein the sensorchip (1) has at least one surface (27) past which the flowing mediumflows; and the sensor area (17) and the additional heater (39) aresituated together on a surface (27).
 17. The method as recited in claim3, wherein the sensor chip (1) supplies a measuring signal for a controlunit of an internal combustion engine; and the additional heater (39) isheated only when the engine is not in operation.
 18. The method asrecited in claim 17, wherein the signal for the heating operation of theadditional heater (39) is supplied by a first control circuit (54) ofthe sensor chip (1).
 19. The method as recited in claim 17, wherein thesignal for heating operation of the additional heater (39) is suppliedby a second control circuit (57).
 20. The method as recited in claim 17,wherein the second control circuit (57) is an engine regulating unit.